Remove some remnants of x2p, which was removed by commit e5a8a0fbd7

Remove some remnants of x2p, which was removed by commit e5a8a0fbd7

References to PERL_FOR_X2P probably have to stay in case any third-party
code is #defining it. Likewise, NO_PERL_TYPEDEFS in vms/vmsish.h has to
stay because it actually is used by App::a2p.

Cross/Makefile.SH.patch still needs regenerating since it contains
references to x2p in the top-level Makefile.SH which were removed by
e5a8a0fbd7.

A few other references to x2p/a2p/find2p/s2p still remain, which I am less
sure about removing, plus some others in documentation which should stay
anyway.